CBSE Class 12 Physics Dual Nature Of Radiation And Matter Notes Set B

 Physics

Ch : Dual Nature of Radiation and Matter

1 Electric Discharge: The passage of an electric current through a gas is called electric discharge.

2 Discharge Tube: A hard glass tube along with the necessary arrangement, which is used to study the passage of electric discharge through gases at low pressure, is called a discharge tube.

3 Cathode Rays. Cathode rays are the stream of negatively charged particles, electrons which are shot out at a high speed from the cathode of a discharge tube at pressure below 0.01 mm of Hg.

4 Work Function. The minimum amount of energy required by an electron to just escape from the metal surface is known as work function of the metal.

5 Electron Emission. The minimum amount of energy required by an electron to just escape from the metal surface is known as work function of the metal.

(i) Thermionic emission. Here electrons are emitted from the metal surface with the help of thermal energy.

(ii) Field or cold cathode emission. Electrons are emitted from a metal surface by subjecting it to a very high electric field.

(iii) Photoelectric emission. Electrons emitted from a metal surface with the help of suitable electromagnetic radiations.

(iv) Secondary emission. Electrons are ejected from a metal surface by striking over its fast moving electrons.

6 Forces Experienced by an Electron in Electric and Magnetic Fields.

(a) Electric field: The force FE experienced by a electron e in an electric field of strength (intensity) E is given by FE = eE

(b) Magnetic field: The force experienced by an electron e in a magnetic field of strength B weber/m2 is given by FB=Bev where v is the velocity with which the electron moves in the electric field and the magnetic field, perpendicular to the direction of motion.

If the magnetic field is parallel to the direction of motion of electron, then, FB = 0.

7 Photoelectric Effect: The phenomenon of emission of electrons from the surface of substances (mainly metals), when exposed to electromagnetic radiations of suitable frequency, is called photoelectric effect and the emitted electrons are called photoelectrons.

8 Cut Off or Stopping Potential: The value of the retarding potential at which the photoelectric current becomes zero is called cut off or stopping potential for the given frequency of the incident radiation.

9 Threshold Frequency: The minimum value of the frequency of incident radiation below which the photoelectric emission stops altogether is called threshold frequency.

10 Laws of Photoelectric Effect.

(i) For a given metal and a radiation of fixed frequency, the number of photoelectrons emitted is proportional to the intensity of incident radiation.

(ii) For every metal, there is a certain minimum frequency below which no photoelectrons are emitted, howsoever high is the intensity of incident radiation. This frequency is called threshold frequency.

(iii) For the radiation of frequency higher than the threshold frequency, the maximum kinetic energy of the photoelectrons is directly proportional to the frequency of incident radiation and is independent of the intensity of incident radiation.

(iv) The photoelectric emission is an instantaneous process.

11 Einstein’s Theory of Photoelectric Effect. Einstein explained photoelectric effect with the help of Planck’s quantum theory. When a radiation of frequency  is incident on a metal surface, it is absorbed in the form of discrete packets of energy called quanta or photons. A part of energy hν of the photon is used in removing the electrons from the metal surface and remaining energy is used in giving kinetic energy to the photoelectron. Einstein’s photoelectric equation is

KE = 1/2MV² = h_v-w₀

where wo is the work function of the metal. If 0 is the threshold frequency, then wo = ho

KE = 1/2mv² = h( v-v₀)

All the experimental observations can be explained on the basis Einstein’s photoelectric equation.

12 Compton Scattering. It is the phenomenon of increase in the wavelength of X-ray photons which occurs when these radiations are scattered on striking an electron. The difference in the wavelength of scattered and incident photons is called Compton shift, which is given by    

Δλ = h / m0c (1-cosφ )

where φ is the angle of scattering of the X-ray photon and m0 is the rest mass of the electron. 

where V = Potential difference between the two electrodes (i.e., P and Q)

d = distance between the two electrodes

R = radius of circular arc in the presence of magnetic field B

l = shift of the electron beam on the screen

 = length of the field

L = distance between the centre of the field and the screen.

14 Milliken’s Oil Drop Method. This determines the charge on the electron. Let ρ be the density of oil, σ is the density of the medium in which oil drop moves and  the coefficient of viscosity of the medium, then the radius r of the drop is where v0 is the terminal velocity of the drop under the effect of

gravity alone. At the terminal velocity v0, the force due to viscosity becomes equal to the electric weight of the body. The charge on oil drop is

where v1 is the terminal velocity of the drop under the influence of electric field and gravity and E is the applied electric field.

15 Photocell. It is an arrangement which converts light energy into electric energy. It works on the principle of photoelectric effect. It is used in cinematography for the reproduction of sound.

16 Dual Nature of Radiation: Light has dual nature. It manifests itself as a wave in diffraction, interference, polarization, etc.,

while it shows particle nature in photoelectric effect, Compton scattering, etc.

17 Dual Nature of Matter: As there is complete equivalence between matter (mass) and radiation (energy) and the principle of symmetry is always obeyed, de Broglie suggested that moving particles like protons, neutrons, electrons, etc., should be associated with waves known as de Broglie waves and their wavelength is called de Broglie wavelength. The de Broglie wavelength of a particle of mass m moving with velocity v is given by

where h is Planck’s constant.

18 Davison and Germer Experiment. This experiment confirms the existence of de Broglie waves associated with electrons.

19 de Broglie Wavelength of an Electron. The wavelength associated with an electron bean accelerated through a potential

20 Electron Microscope: It is a device which makes use of accelerated electron beams to study very minute objects like viruses, microbes and the crystal structure of solids. It has a magnification of ˜ 10^5.

TOP Formulae

1. Maximum kinetic energy of the photoelectrons emitted from the metal surface: max Kmax (Einstein’s Photoelectric equation)

2. Work function of a metal surface:

w_o =Φ_o =h_o 

3. de Broglie wavelength associated with the particle of momentum p is

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